Tubular segments are often joined together to form tubular strings for use in oilfield applications. Tubular strings tend to require high strength and fatigue resistance as well as high fracture toughness. Strength is the maximum amount of stress a component can withstand without failure. Fatigue resistance is the ability of a component to resist fatigue failure, for example crack propagation. Fracture toughness is the resistance to failure from a fracture originating from a preexisting crack. Other desirable properties include fatigue resistance, strength, fracture toughness, fatigue life, corrosion resistance, hardness, resistance to weld cracking, and/or anti-galling ability.
Tubulars for oilfield applications are typically joined in situ. For example, a tubular string may be joined at a wellhead that is in the vicinity of the entrance to a well. The well has a borehole extending into a formation. The tubular string is lowered through a rig floor into the borehole and suspended at the rig floor using a suspending device, such as a spider. The proximal end of the pipe string is positioned above the suspending device to facilitate the joining of additional tubular segments to the tubular string, after which the lengthened tubular string is lowered further through the rig floor. This process is repeated until the tubular string reaches the desired length for being installed in the borehole.
A joining process for tubular strings may be a mechanical joining process, a metallurgical joining process, or a combination of any suitable processes. For example, in one type of mechanical joining process tubular segments are threadedly connected. The ends of a pair of the tubular segments to be joined are equipped with threads. Although threading is one type of joining process, there has been interest in other alternatives, such as welding.
Welding is an example of a metallurgical joining process. Welding tubular segments involves bringing the ends of a pair of tubular segments to be joined into contact or proximity and applying or creating heat. Through the welding process the heated ends are joined. Some types of welding processes, but not all, involve a filler material that assists in the weld and is incorporated in the joint.
A joining process, such as welding, can result in tensile residual stresses in the joined component. Tensile stress is stress that causes two regions of a component on either side of a plane dividing the two regions to elongate. These tensile residual stresses are capable of reducing desirable properties, such as fatigue resistance, strength, fracture toughness, fatigue life, corrosion resistance, hardness, resistance to weld cracking, and/or anti-galling ability. Fatigue life describes the number of cycles at which a component fails under cyclic loading. Examples of strength are tensile strength and yield strength. Tensile strength is the ability to withstand tensile stress. Yield strength is the maximum stress a component can withstand without deforming permanently.
Laser shock peening is a known technique for inducing compressive stress in a work piece. Compressive stress is the opposite of tensile stress. That is, it is stress that causes two regions of a component on either side of a plane dividing the regions to contract. Laser shock peening is also termed herein laser peening. Laser peening relies on the production and propagation of a shockwave to generate plastic deformation of a work piece. During laser peening, an opaque overlay material may be applied to the region designated to be laser peened. The opaque overlay material serves as an ablative material. A second, transparent overlay material, generally water, is allowed to flow over the designated region. The transparent overlay material serves as a confinement material. The designations of opaque and transparent refer to opacity and transparency with respect to the coherent laser radiation. For example, coherent radiation from a neodymium laser with water as the transparent material and black paint as the opaque material may be used in a laser peening process.
Once the laser peening process has begun, a pulse of coherent laser radiation is allowed to pass through the transparent overlay and rapidly vaporizes at least a portion of the opaque overlay, or at least a portion of the base material if the opaque overlay is omitted, as it absorbs the radiation. The vaporization creates a high pressure, rapidly expanding plasma which is confined by the transparent overlay material. This confinement of the plasma, known as a confined ablation mode, causes the high pressure shockwave to propagate through the material. Plastic deformation results if the pressure of the shockwave exceeds the dynamic yield strength of the material. Compressive residual stresses directly proportionate to the degree of deformation may be readily achieved.
Tensile residual stresses introduced by tubular joining processes are located proximate to the joint between adjacent tubular segments. Tubular segments, typically 30 ft. to 60 ft. (9.1 m to 18.3 m) in length, can be joined together to form a tubular string up to 10,000 ft. (3,048 m) or more. Thus a joint region may be located a distance of 30 ft. or greater from the open end of the tubular string.